HELP: Upgrading an SM-G900A to Lollipop in Canada

I recently bought an SM-G900A. It came with 4.4.2 (NOT 4.4.4) and it is the AT&T version. I have read that you must have an active SIM card on the AT&T network to upgrade via OTA, or get a US BestBuy to do it for you. Both are out of the question for now since I live in Canada.

Am I stuck with KitKat or is there a way to update it to Lollipop?
I don't want to root the device or lose my warranty.

(When I tried to check for updates via software update it said something about 'software update interrupted','you are running the latest firmware' and 'try again in 24 hours')